Re: Attribute Setting Methods
On Jan 16, 4:41 am, "Daniel T." <danie...@earthlink.net> wrote:
"kj...@cablescan.com" <kj...@cablescan.com> wrote:
I wonder if someone may have a solution to a small problem I'm
experiencing with a C++ program? I'm have two classes in my
application that provides methods for setting parameters in the
classes. These methods returns a pointer to the class itself so that
you can use the return value to call another attribute setting method
on the same line.
This isn't SmallTalk... There are situations where this sort
of idiom is useful in C++, this isn't one of them. Make your
setters return 'void'.
I'm not sure why. There are three common idioms for setters in
C++:
-- Returning void. This is the simplest for the implementor.
-- Returning a reference to self. This allows chaining:
obj.setA( newA ).setB( newB )...
-- Returning the old value. This allows restoring it by the
user:
A oldA = obj.setA( newA ) ;
// ...
obj.setA( oldA ) ;
Of the three, the first is probably the least useful.
His problem is really a design problem. Client interfaces in
C++ should be complete: if the derived class wants to support
setA, in addition to setB, then it really has to declare it. In
some cases, it's a bit awkward, but the additional type safety
provided by static checking is worth it. So his derived class
would become something like:
class Sub : public Base
{
public:
Sub& setB( int newB ) { /* ... */ }
Sub& setA( int newA )
{
Base::setA( newA ) ; return
*this ;
}
// ...
} ;
Obviously, if Base is a fairly complex interface, and Sub just
adds one simple function, this can be a bit painful, but it's
really the only clean solution.
(For the original poster: you really should return a reference
when chaining---it can't be null. And Microsoft uses the
convention of CXxx for classes in MFC, so you should probably
avoid it, to avoid risking a collision if ever your code is used
with MFC.)
--
James Kanze (GABI Software) email:james.kanze@gmail.com
Conseils en informatique orient=E9e objet/
Beratung in objektorientierter Datenverarbeitung
9 place S=E9mard, 78210 St.-Cyr-l'=C9cole, France, +33 (0)1 30 23 00 34